Adaptive pulse shaping method and apparatus for unlatching a VCM in a disc drive

ABSTRACT

A method and apparatus for unlatching a VCM in a disc drive is provided in which a determination is made if the VCM is stationary. A first unlatch current pulse that has a first amplitude and first width is applied to the VCM if the VCM is found to be stationary. An incremented unlatch current pulse is then applied to the VCM if the VCM is found to be stationary after application of the first unlatch current pulse. The incremented unlatch current pulse has at least one of an amplitude and a width that is greater than at least one of the corresponding first amplitude and the first width of the first unlatch current pulse.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims benefit of U.S. Provisional ApplicationNo. 60/224,179, filed Aug. 9, 2000, and entitled “ADAPTIVE PULSE SHAPINGMETHOD FOR UNLATCH IN HARD DISC DRIVE”.

FIELD OF THE INVENTION

[0002] The present invention relates to voice coil motors (VCM) in discstorage systems. In particular, the present invention relates to anadaptive pulse-shaping method for unlatching a VCM in a disc storagesystem.

BACKGROUND OF THE INVENTION

[0003] In disc drives, a VCM is used to position the transducer headsover a desired radial position on a stack of discs that storeinformation. When the disc drive is energized and the discs arespinning, the VCM positions the heads over data stored on the spinningdiscs. When the disc drive is de-energized, the discs stop spinning, andthe heads with the VCM are moved to a “park” position of the discs.Typically, no data is stored on the park position. Various kinds oflatches are used to latch the VCM in the park position when the discdrive is de-energized. A latching mechanism can be a magnetic latch orany other form of mechanical latch.

[0004] Unlatching the VCM is one of the first tasks for the disc driveunder a cold start condition. In this condition, the latch force may bea combination of the designed latch force of the latching mechanism, theforce associated with sticking friction (“stiction”) between the headsand the disc, a bias force applied by a flexible head suspension, etc.These forces vary from one drive to another. Further, the VCM actuatorsystem parameters such as the VCM torque constant, K_(t), can be changedamong the drives and as the drive operational environment changes. Thelatch force can even vary between each unlatch operation because ofvariations in stiction. In spite of these variations, unlatch operationsmust always be smooth to prevent damage to the disc and to help ensureproper read/write operations.

[0005] One technique for unlatching a VCM is to apply an open loopkick-off current pulse. However, since the latch force can vary widely,applying the same open loop kick-off current pulse for each unlatchoperation will not result in a smooth transfer from open loop control tothe feedback control. Also, excessive head velocity overshoot may occur,resulting in the head colliding with the disc medium if the kick-offcurrent pulse method is used in a ramp load drive.

[0006] Another unlatch technique is described in U.S. Pat. No. 5,600,219entitled “SENSORLESS CLOSED-LOOP ACTUATOR UNLATCH”. This unlatch schemedeals with adjusting unlatch current amplitude incrementally, followedby the application a negative current pulse to decelerate the VCM assoon as the unlatch is confirmed. This system does not take intoconsideration the possibility of varying the width of unlatch currentpulses to optimize the unlatch process. Further, final pulse width andamplitude values of a final unlatch current pulse from a previousunlatch operation are not stored for reference in the next unlatchoperation. Thus, this unlatch technique does not guarantee a smoothunlatch process.

[0007] The present invention addresses these problems, and offers otheradvantages over the prior art.

SUMMARY OF THE INVENTION

[0008] The present embodiments relate to disc storage systems thatemploy an adaptive pulse-shaping scheme for unlatching a VCM in a discdrive, thereby addressing the above-mentioned problems.

[0009] One embodiment relates to a method for unlatching a VCM in a discdrive. The method includes determining if the VCM is stationary andapplying a first unlatch current pulse that has a first amplitude andfirst width to the VCM if the VCM is found to be stationary. Anincremented unlatch current pulse is then applied to the VCM if the VCMis found to be stationary after application of the first unlatch currentpulse. The incremented unlatch current pulse has at least one of anamplitude and a width that is greater than at least one of thecorresponding first amplitude and the first width of the first unlatchcurrent pulse.

[0010] Another embodiment relates to a disc storage system that includesa VCM velocity control loop operating a VCM and an adaptive currentpulse shaping means coupled to the VCM velocity control loop. Theadaptive current pulse shaping means is capable of providing unlatchcurrent pulse for unlatching the VCM.

[0011] These and various other features as well as advantages whichcharacterize the present invention will be apparent upon reading of thefollowing detailed description and review of the associated drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a perspective view of an exemplary magnetic disc drivein which the present invention can be used.

[0013]FIG. 2 is a block diagram of a VCM velocity control loop employingthe adaptive current pulse shaping circuit of the present invention.

[0014]FIG. 3 is a simplified flow diagram of the present invention.

[0015]FIG. 4-1 is a flow diagram of an incremental pulse shapingalgorithm (IPSA) of the present invention.

[0016]FIG. 4-2 is an example of incremental pulse shaping by the IPSA ofFIG. 4-1.

[0017]FIG. 5-1 is a flow diagram of a decremental pulse shapingalgorithm (DPSA) of the present invention.

[0018]FIG. 5-2 shows example of decremental pulse shaping by the DPSA ofFIG. 5-1.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0019] Referring now to FIG. 1, a perspective view of a magnetic oroptical disc drive 100 with which the present invention is useful isshown. Disc drive 100 includes a housing with a base 102 and a top cover(not shown). Disc drive 100 further includes a disc pack 106, which ismounted on a spindle motor (not shown), by a disc clamp 107. Disc pack106 includes a plurality of individual discs, which are mounted forco-rotation about central axis 109. Each disc surface has an associatedtransducer head 110, which is mounted on a head assembly 112 mounted todisc drive 100 for communication with the disc surfaces. Transducer head110 can include a read head, a write head, or both a read and a writehead. Each disc surface further includes a park surface 114, which istextured, for parking transducer head 110 when the disc drive 100 isde-energized. Head assembly 112 is mounted to a bearing 122 which isactuated to rotate or pivot on an axle shaft 116, or other pivot point,which is mounted to the base 102. A VCM, shown generally at 118,actuates a voice coil 120 that is attached to the bearing 122. VCM 118rotates bearing 122 with its attached head assembly 112 about the axleshaft 116 to position transducer head 110 over a desired data trackalong an arcuate path 124 between a disc inner diameter 122 and a discouter diameter 128. VCM 118 operates under control of internal circuitry130. When disc drive 100 is de-energized, the VCM 118 moves the head tothe park surface 114, and a latch, shown generally at 132, latches theVCM in a position that parks the transducer head 110 on the park surface114. Various kinds of latches are used to latch VCM 118 in the parkposition when the disc drive is de-energized. A latching mechanism canbe a magnetic latch or any other form of mechanical latch.

[0020] Under the present invention, an adaptive current pulse-shapingscheme is employed for unlatching VCM 118. This scheme involves theapplication of a series of amplitude and pulse-width adjusted unlatchcurrent pulses that cope with the variation of latch torque amplitudecaused by variations in stiction, changes in latch force distribution,variation of torque constant, K_(t), etc. A VCM velocity control loopcontained in internal circuitry 130 includes the adaptive currentpulse-shaping circuit of the present invention.

[0021] Referring now to FIG. 2, a block diagram of a VCM velocitycontrol loop 200 employing an adaptive current pulse shaping module orcircuit 202 of the present invention is shown. Loop 200 includes atransconductance amplifier 208 that provides initial demand current tounlatch VCM 118 from a stationary to an energized state.Transconductance amplifier 208 also supplies demand current to drive theVCM 118 when the disc drive is energized and the discs are spinning.

[0022] Digital controlling code (hereinafter amplitude and pule widthparameters) from either adaptive current pulse shaping circuit 202(during unlatch operations) or velocity feedback controller 240 (whenthe VCM is energized) is converted to analog input voltage signals fortransconductance amplifier 208 by digital to analog converter (DAC) 206.Switch 204 connects either adaptive current pulse shaping circuit 202 orvelocity feedback controller 240 to DAC 206. Based on the amplitude andpulse width parameters, transconductance amplifier 208 outputs VCMunlatch/drive current (i_(L)) 210 which produces VCM driving torque (τ)214. Block 212 represents VCM torque constant K_(t). Torque disturbance(τ_(d)) 216, which exists inherently in the system, is combined with τat summing node 218. Block 220 represents VCM arm inertia (1/J_(m)). VCMangular acceleration (a) 222 is integrated at block 224 to produce VCMangular velocity (ω) 226 having units of radians/second. Angularvelocity, ω, is integrated at block 228 to provide VCM position (θ) 230in radians. In block 231, angular velocity, ω, is multiplied by back emfcoefficient, K_(e), shown by block 232 and fed to analog to digitalconverter (ADC) 233 that outputs back emf voltage (V_(bemf)) 234 in adigital form which is monitored to examine movement of VCM 118. V_(bemf)is fed to adaptive current pulse shaping circuit 202 and also deductedfrom reference voltage (Vref) 236 at summing node 238. The output ofsumming node 238 is fed to velocity feedback controller 240 whichprovides fast and damped VCM velocity control when movement of the VCMis confirmed after the completion of an unlatch operation.

[0023] The method of operation of the adaptive current pulse shapingcircuit 202 of the VCM velocity control loop 200 is described below withthe help of flow diagrams 300, 400 and 500.

[0024] Referring now to FIG. 3, a simplified flow diagram 300illustrating a method of unlatching a VCM of a disc drive in accordancewith the present invention is shown. The method shown in flow diagram300 begins at state 302 and proceeds to state 304 where a determinationis made if the VCM is stationary. In state 306, a first unlatch currentpulse that has a first amplitude and first width is applied to the VCMif the VCM is found to be stationary. The method continues at state 308where an incremented unlatch current pulse is then applied to the VCM ifthe VCM is found to be stationary after application of the first unlatchcurrent pulse. The incremented unlatch current pulse has at least one ofan amplitude and a width that is greater than at least one of thecorresponding first amplitude and the first width by an incrementalvalue. A series of incremented current pulses may be applied until theVCM unlatches. The process ends at state 310.

[0025] Algorithms to increase and decrease the amplitude and pulse-widthparameters while performing the unlatch operation in states 306 and 308(of FIG. 3) are described below.

[0026] Referring now to FIG. 4-1, a flow diagram of an IPSA of thepresent invention is shown. The method shown in flow diagram 400 isperformed for one or more iterations. Flow diagram 400 begins at state402 and proceeds to state 404 where a minimum or initial amplitude,h_(min), and a minimum or initial pulse-width, w_(min), are set for aninitial unlatch current pulse. At state 406, an unlatch current pulseI_(p) with amplitude h_(n) (h_(n)=h_(min) for the first iteration, h₁,h₂, etc., for subsequent iterations) and width w_(n) (w_(n)=w_(min) forthe first iteration, w₁, w₂, etc., for subsequent iterations) is appliedto the VCM. At state 408, the VCM is disabled to read its back emfvoltage, V_(bemf), which is used to monitor VCM movement.

[0027] In state 410, V_(bemf) is compared with a threshold value ofV_(bemf) (α) for indication of VCM movement. If V_(bemf) is greater thanthe threshold value, α, then the unlatch operation terminates and theVCM operation switches to feedback control at state 412.

[0028] If V_(bemf) is below cc in state 410, then the unlatch currentpulse amplitude is increased from the amplitude of the unlatch currentpulse of the previous iteration, h_(n−1), by an incremental amplitudevalue, Δ₁, to provide a current iteration amplitude, h_(n)=h_(n−1)+Δ₁,in state 414.

[0029] In state 416, h_(n) is compared with a maximum amplitude value,h_(max). If h_(n) is less than h_(max) in state 416, then states 406,408, 410, 414 and 416 repeated until either V_(bemf) is greater than αat state 410 or h_(n) is greater than h_(max) at state 416 in asubsequent iteration.

[0030] If h_(n) is greater than h_(max) at state 416, then at state 418,h_(n) is set equal to h_(max) and the current pulse width is increasedfrom the unlatch current pulse of the previous iteration, w_(n−1), by anincremental pulse-width value, Δ₂, to provide a current iterationpulse-width w_(n)=w_(n−1)+Δ₂.

[0031] States 406, 408, 410, 414, 416 and 418 are repeated until the VCMis unlatched (V_(bemf) is greater than α at state 410).

[0032] Referring now to FIG. 4-2, an example of incremental pulseshaping by the IPSA of FIG. 4-1 is shown. Plot 450 shows the variationof unlatch current along vertical axis 452 as a function of time alonghorizontal axis 454. An initial unlatch current pulse 456, applied bythe IPSA, has an initial or minimum amplitude 458 (h_(min)) and aninitial or minimum pulse width 460 (w_(min)). Time duration 462 is theperiod during which the back emf of the VCM (V_(bemf)) is monitored. Asecond unlatch current pulse 464 is applied after V_(bemf) is monitored.The amplitude of second pulse 464 is greater than that of initial pulse456 by incremental amplitude value 466 (Δ₁). The amplitudes of pulses468 and 470 are similarly increased while V_(bemf) is monitored betweeneach pulse. Pulse 472 has a maximum amplitude 478 (h_(max)), but theapplication of this maximum amplitude pulse does not result in the VCMbeing unlatched.

[0033] Since a pulse with h_(max) has been applied and the VCM is stillnot unlatched, pulse 474 is increased in width by an incremental pulsewidth value 480 (Δ₂). The VCM is unlatched when pulse 476 is applied.Pulse 476 has a width increased by 482 (2Δ₂) from w_(min). The unlatchis confirmed at time 484 by the V_(bemf) monitoring process. Thisresults in termination of the unlatch process and the velocity feedbackcontroller taking control of the VCM. Pulses 486 and 488 are variationsof VCM current with the velocity feedback controller driving the VCM.The amplitude and width of pulse 476 is saved at the end of the unlatchoperation.

[0034] Referring now to FIG. 5-1, a DPSA in accordance with the presentinvention is shown. The method illustrated if flow diagram 500 isessentially the method of flow diagram 400 (IPSA) performed in reverseorder. Amplitude and pulse-width values from a previous unlatchoperation which were stored serve as the initial amplitude and pulsewith values for the DPSA. The pulse-width is first decreased in stepsfor each iteration until it reaches a minimum value and then theamplitude is decreased for each additional iteration. During eachiteration the VCM is monitored for movement. These steps are thereforein reverse order of IPSA flow diagram 400 which involved increasingamplitude in steps and then increasing pulse-width in steps. Details offlow diagram 500 are described below.

[0035] The method illustrated in flow diagram 500 is performed for oneor more iterations. Flow diagram 500 begins at state 502 and proceeds tostate 504 where an unlatch current pulse amplitude is set to a previousamplitude value, h_(n−1) and a previous pulse-width value, w_(n−1),stored at the end of a previous unlatch operation. In state 506, w_(n)is compared with minimum pulse-width value w_(min). If w_(n) is greaterthan w_(min), then the unlatch current pulse-width is decreased from thepulse-width of the unlatch current pulse of the previous iteration,w_(n−1), by an decremental pulse width value, Δ₂, to provide a currentiteration pulse-width, w_(n)=w_(n−1)−Δ₂, in state 508. In state 510, anunlatch current pulse I_(p) with amplitude h_(n) and widthw_(n)(w_(n−1)−Δ₂) is applied to the VCM.

[0036] At state 512, the VCM is disabled to read its back emf voltage,V_(bemf). In state 514, V_(bemf) is compared with a threshold value ofV_(bemf) (α) for indication of VCM movement. If V_(bemf) is greater thanthe threshold value, α, then the unlatch operation terminates and theVCM operation switches to feedback control at state 516.

[0037] If V_(bemf) is below α in state 514, then steps 506, 508, 510,512 and 514 are repeated until either V_(bemf) is found to be greaterthan α at state 514 or w_(n) is less than w_(min) at state 506.

[0038] If w_(n) is less than w_(min) at state 506, then at state 518,w_(n) is set equal to w_(min) and the unlatch current pulse amplitude isdecreased from the unlatch current amplitude of the previous iteration,h_(n−1), by a decremental pulse-width value Δ₁(h_(n)=h_(n−1)−Δ₁) instate 520.

[0039] States 506, 518, 520, 510, 512 and 514 are repeated until the VCMis unlatched (V_(bemf) is greater than α at state 514) or until theamplitude of the unlatch current pulse has reached a minimum valueh_(min). If the VCM is still not unlatched at h_(min), then the DPSA isterminated and the unlatch operation continues with the application ofthe IPSA.

[0040] Referring now to FIG. 5-2, examples of decremental pulse shapingby the DPSA of FIG. 5-1 is shown. Plot 550 shows the variation ofunlatch current along vertical axis 552 as a function of time alonghorizontal axis 554. Plot 550 includes three separate unlatch operationsperformed by the DPSA algorithm. The first unlatch operation performedby the DPSA includes the application of pulse 556 which has, forexample, a amplitude equal to maximum amplitude value 558 (h_(max)) anda pulse-width equal to minimum pulse width value 560 (w_(min)) plusincremental pulse width value 562 (Δ₂). The first unlatch is confirmedat time 564 by the V_(bemf) monitoring process and the amplitude andpulse width values are saved. This results in termination of the unlatchprocess and the velocity feedback controller taking control of the VCM.Pulses 566 is deceleration pulse applied the velocity feedbackcontroller.

[0041] Pulse 568 is applied at the beginning of a second unlatchoperation. Pulse 568 has an amplitude 558 of h_(max) and a pulse-width570 of w_(min) which is less than the pulse width of the first unlatchoperation by Δ₂. The unlatch is confirmed by the VCM monitoring processat time 572 and the second unlatch operation is terminated. Pulse 574 isa deceleration pulse similar to pulse 566 but not as strong because thePulse 568 has a width that is less than pulse 556 and therefore hasreduced overshoot.

[0042] In the third unlatch operation a pulse with an amplitude lessthan h_(max) (the amplitude value of pulses 566 and 568) by value 578(Δ₁) is applied to the VCM. Again, the unlatch is confirmed by the VCMmonitoring process at time 580 and the third unlatch operation isterminated. A very small deceleration pulse 582 is applied because ofthe decreased strength of pulse 576 compared to pulses 556 and 568.

[0043] In summary, a method embodiment of unlatching a VCM in a discdrive storage system includes a state (such as 304) for determining ifthe voice coil motor is stationary. In a second state (such as 306) afirst unlatch current pulse that has a first amplitude and first widthis applied to the VCM if the VCM is found to be stationary. In a thirdstate (such as 308) an incremented unlatch current pulse is applied tothe VCM if the VCM is found to be stationary after application of thefirst unlatch current pulse. The incremented unlatch current pulse hasat least one of an amplitude and a width that is greater than at leastone of the corresponding first amplitude and the first width by anincremental value.

[0044] In a disc storage system embodiment a VCM velocity control loop(such as 200) includes a transconductance amplifier (such as 208) thatdrives a VCM (such as 118) and a back emf sensing circuit (such as 233)that monitors movement of the VCM (such as 118). Loop (such as 200) alsoincludes an adaptive current pulse shaping circuit (such as 202) adaptedto apply a first unlatch current pulse that has a first amplitude andfirst width to the VCM (such as 118) if the VCM (such as 118) is foundto be stationary by the back emf sensing circuit (such as 233), and toapply incremented unlatch current pulse to the VCM (such as 118) if theVCM (such as 118) is found to be stationary by the back emf sensingcircuit (such as 233) after application of the first unlatch currentpulse. The incremented unlatch current pulse has at least one of anamplitude and a width that is greater than at least one of thecorresponding first amplitude and the first width by an incrementalvalue.

[0045] It is to be understood that even though numerous characteristicsand advantages of various embodiments of the invention have been setforth in the foregoing description, together with details of thestructure and function of various embodiments of the invention, thisdisclosure is illustrative only, and changes may be made in detail,especially in matters of structure and arrangement of parts within theprinciples of the present invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed. For example, the particular elements may vary depending onthe particular application for the disc storage system while maintainingsubstantially the same functionality without departing from the scopeand spirit of the present invention. In addition, although the preferredembodiment described herein is directed to an adaptive pulse shapingscheme for unlatching a VCM for a disc storage system, it will beappreciated by those skilled in the art that the teachings of thepresent invention can be applied to other systems, without departingfrom the scope and spirit of the present invention. Further, theadaptive pulse shaping scheme may be implemented in hardware instead ofsoftware without departing from the scope and spirit of the presentinvention. Other modifications can also be made. The disc drive can bebased upon magnetic, optical, or other storage technologies and may ormay not employ a flying slider.

What is claimed is:
 1. A method of unlatching a voice coil motor in adisc drive, the method comprising: (a) determining if the voice coilmotor is stationary; (b) applying a first unlatch current pulse to thevoice coil motor, the first unlatch current pulse having a firstamplitude and a first width, the first unlatch current pulse beingapplied if the voice coil motor is found to be stationary in accordancewith step (a); and (c) applying an incremented unlatch current pulse tothe voice coil motor, the incremented unlatch current pulse having anamplitude at least as large as the first amplitude and a width greaterthan the first width by an incremental value, the incremented unlatchcurrent pulse being applied if the voice coil motor is found to bestationary in accordance with step (a) after applying the first unlatchcurrent pulse in accordance with step (b).
 2. The method of claim 1further comprising repeating step(c) iteratively with each iterationapplying a current iteration incremented unlatch pulse to the voice coilmotor, the current iteration incremented unlatch pulse having at leastone of a current iteration amplitude and a current iteration widthgreater than at least one of a corresponding amplitude and a width of anincremented unlatch current pulse of an immediately previous iterationby the incremental value, the iterations being repeated until the voicecoil motor moves.
 3. The method of claim 2 further comprising storing afinal amplitude and a final width of a final unlatch current pulse thatcauses the voice coil motor to move.
 4. The method of claim 3 furthercomprising using the stored final amplitude and the final width asparameters for a subsequent unlatch operation.
 5. The method of claim 1wherein the incremented unlatch current pulse of step(c) is anincremented amplitude current pulse having an incremented amplitude thatis greater than the first amplitude by an incremental amplitude valueand a width that is equal to the first width.
 6. The method of claim 5further comprising repeating step (c) iteratively with each iterationapplying a current iteration incremented amplitude unlatch pulse havinga current iteration amplitude that is greater than an amplitude of anincremented amplitude unlatch current pulse of an immediately previousiteration by the incremental amplitude value, the iterations beingrepeated if the voice coil motor is found to be stationary by thedetermining step (a) until a maximum amplitude current pulse is applied.7. The method of claim 6 further comprising applying an incrementedwidth current pulse after the maximum amplitude current pulse is appliedand the voice coil motor is found to be stationary by the determiningstep (a), the incremented width current pulse having an incrementedwidth that is greater than the first width by an incremental width valueand an amplitude equal to a maximum amplitude of the maximum amplitudecurrent pulse.
 8. The method of claim 7 further comprising repeatingstep (c) iteratively after applying the incremented width current pulsewith each iteration applying a current iteration incremented widthunlatch pulse having a current iteration width that is greater than awidth of an incremented width unlatch current pulse of an immediatelyprevious iteration, the iterations being repeated until the voice coilmotor moves.
 9. The method of claim 1 wherein the determining step (a)is performed by monitoring the voice coil motor for movement bymeasuring a change in back emf voltage of the voice coil motor.
 10. Themethod of claim 1 wherein the first unlatch current pulse is an initialcurrent pulse having a minimum amplitude and a minimum width.
 11. Themethod of claim 1 wherein the first unlatch current pulse is adecremented unlatch current pulse having at least one of an amplitudeand a width less than at least one of a corresponding one of a finalamplitude and a final width of a final unlatch current pulse by adecremental value, the final amplitude and final width being storedparameters of a final unlatch current pulse that caused the voice coilmotor to move at the end of a previous unlatch operation.
 12. The methodof claim 11 wherein the decremented unlatch current pulse is adecremented width current pulse having a decremented width that is lessthan the final width by a decremental width value and an amplitude equalto the final amplitude value.
 13. The method of claim 12 wherein thedecremented width is maintained above a minimum width value.
 14. Themethod of claim 13 wherein the decremented width is set equal to theminimum width value if a computed decremented width is less than theminimum width value.
 15. The method of claim 14 wherein the decrementedunlatch current pulse is a decremented amplitude current pulse having adecremented amplitude that is less than the final amplitude by adecremental amplitude value, and wherein the decremented amplitudecurrent pulse is applied when the decremented width is set equal to theminimum width value.
 16. The method of claim 15 wherein the decrementedamplitude is maintained above a minimum amplitude value.
 17. The methodof claim 16 wherein step (c) is performed if a computed decrementedamplitude is less than or equal to the minimum amplitude value.
 18. Adisc storage system having a voice coil motor velocity control loopcomprising: a transconductance amplifier driving a voice coil motor; aback emf sensing circuit for detecting movement of the voice coil motor;and an adaptive current pulse shaping circuit adapted to: apply a firstunlatch current pulse to the voice coil motor, the first unlatch currentpulse having a first amplitude and a first width, the first unlatchcurrent pulse being applied if the voice coil motor is found to bestationary by the back emf sensing circuit; and to apply an incrementedunlatch current pulse to the voice coil motor, the incremented unlatchcurrent pulse having at least one of an amplitude and a width greaterthan at least one of the corresponding first amplitude and the firstwidth by an incremental value, the incremented unlatch current pulsebeing applied if the voice coil motor is found to be stationary by theback emf sensing circuit after applying the first unlatch current pulse.19. The disc storage system of claim 18 wherein the back emf sensingcircuit determines movement of the voice coil motor by measuring achange in back emf voltage of the voice coil motor.
 20. The disc storagesystem of claim 18 wherein the adaptive current pulse shaping circuit isfurther adapted to iteratively apply incremental unlatch current pulseswith each iteration applying a current iteration unlatch pulse to thevoice coil motor, the current iteration unlatch pulse having at leastone of a current iteration amplitude and a current iteration widthgreater than at least one of a corresponding amplitude and a width of anunlatch current pulse of an immediately previous iteration by theincremental value, the iterations being repeated until the voice coilmotor moves.
 21. The disc storage system of claim 20 further comprisinga memory for storing a final amplitude and a final width of a finalunlatch current pulse that causes the voice coil motor to move.
 22. Thedisc storage system of claim 21 wherein the stored final amplitude andthe final width are used as parameters for a subsequent unlatchoperation.
 23. A disc storage system comprising: a voice coil motorvelocity control loop operating a voice coil motor; and pulse shapingmeans capable of providing unlatch current pulses adaptively forunlatching the voice coil motor.
 24. The disc storage system of claim 23wherein: the voice coil motor velocity control loop comprising: atransconductance amplifier driving the voice coil motor; and a back emfsensing circuit for detecting movement of the voice coil motor; and thepulse shaping means comprises an adaptive pulse shaping circuit adaptedto: apply a first unlatch current pulse to the voice coil motor, thefirst unlatch current pulse having a first amplitude and a first width,the first unlatch current pulse being applied if the voice coil motor isfound to be stationary by the back emf sensing circuit; and to apply anincremented unlatch current pulse to the voice coil motor, theincremented unlatch current pulse having at least one of an amplitudeand a width greater than at least one of the corresponding firstamplitude and the first width by an incremental value, the incrementedunlatch current pulse being applied if the voice coil mo tor is found tobe stationary by the back emf sensing circuit after applying the firstunlatch current pulse.
 25. The disc storage system of claim 24, whereinthe back emf sensing circuit determines movement of the voice coil motorby measuring a change in back emf voltage of the voice coil motor. 26.The disc storage system of claim 24, wherein the adaptive current pulseshaping circuit is further adapted to iteratively apply incrementalunlatch current pulses with each iteration applying a current iterationunlatch pulse to the voice coil motor, the current iteration unlatchpulse having at least one of a current iteration amplitude and a currentiteration width greater than at least one of a corresponding amplitudeand a width of an unlatch current pulse of an immediately previousiteration by the incremental value, the iterations being repeated untilthe voice coil motor moves.
 27. The disc storage system of claim 26further comprising a memory for storing a final amplitude and a finalwidth of a final unlatch current pulse that causes the voice coil motorto move.
 28. The disc storage system of claim 27 wherein the storedfinal amplitude and the final width are used as parameters for asubsequent unlatch operation.